Field of the Invention
The present invention relates to an automatic transmission for a vehicle.
Description of Related Art
Recent increases in oil prices are triggering stiff competition among auto-makers in enhancing fuel consumption of a vehicle.
Therefore, research is being conducted on engines in terms of reducing weight and improving fuel efficiency by down-sizing, and research is also being conducted to ensure both drivability and competitiveness for maximizing fuel efficiency by implementing an automatic transmission with multiple speed stages.
However, in the case of the automatic transmission, the number of internal components increases as the number of gear shift stages is increased, which may deteriorate installability, production cost, weight and/or power flow efficiency.
Therefore, in order to maximally enhance fuel consumption of an automatic transmission having more shift stages, it is important for better efficiency to be derived by a smaller number of parts.
In this respect, an eight-speed automatic transmission has been recently introduced, and a planetary gear train for an automatic transmission enabling more shift stages is under investigation.
Considering that gear ratio spans of recently developed eight-speed automatic transmissions are typically between 6.5 and 7.5, fuel consumption enhancement is not very large.
In the case of a gear ratio span of an eight-speed automatic transmission having a level above 9.0, it is difficult to maintain step ratios between adjacent shift stages to be linear, by which driving efficiency of an engine and drivability of a vehicle are deteriorated.
Thus, research studies are underway for developing a high efficiency automatic transmission having nine or more speeds.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.